In Q 20 of gravitation (Why does a block of plastic released under water come up to the surface of water?) it is given that the upthrust experienced by the body is greater than the weight of the body. But Archimedes Principle states that the upthrust exprienced by a body partially or wholly immersed in water is equal to the weight of object immersed. Then shouldn't the upthrust be equal to the weight of the body and not more than it?

Archimedes' principle states that " when a body is wholly or partially immersed in a liquid, it experiences a buoyant force or upthrust which is equal to the weight of the liquid displaced by the object" .

It is not like what you have stated: i.e. the upthrust exprienced by a body partially or wholly immersed in water is equal to the weight of object immersed.Both the statement is totally different. The upthrust experienced by a body immersed in liquid is equal to the weight of the liquid displaced by the immersed body.

 

When an object is put in a liquid, two forces acts on it:

1. Weight of the object acting downwards

2.Upthrust exerted by the liquid on the object, acting upwards.

If the upthrust exerted by the liquid  is equal to the weight of the object, then the object will just float in the liquid and if the upthrust exerted by the liquid it is greater than the weight of the object, the object will rise in the liquid and then float.

Hence we can say that when a plastic is released under water, it rises up to the surface of water as the upthrust exerted by the liquid on the plastic block is greater than the weight of the plastic.